Electromagnetic engraving apparatus



Feb. 6, 1962 Filed Jan. 25. 1956 J. B. MINTER 2ND 3,020,361

ELECTROMAGNETIC ENGRAVING APPARATUS 2 Sheets-Sheet 1 -lriclm- L Plz/ok Ala-r BY ATTORN Y Feb. 6, 1962 J. B. MINTER 2ND ELECTROMAGNETIC ENGRAVING APPARATUS 2 Shee'cs-Sheet- 2 Fled Jan. 25. 1956 DRH/ING QMPLIFIEI? NETWORK ELECTRIC SIGNQL SOURCE.

HEATING CURRENT eouca INVENToR. JERRY B. MINTER, 2N.

U ATTORNEY United States Patent G 3,020,361 ELECTROMAGNETIC ENGRAVING APPARATUS Jerry B. Minter, 2nd, Normandy Heights Road, Morristown, NJ.

Filed Jan. 23, 1956, Ser. No. 560,727 20 Claims. (Cl. 179-100.41)

This invention relates to cutting or engraving apparatus, and more particularly it relates to the cutting of record grooves in phonograph blanks and the like.

A principal object of the invention is to provide improved signal-controlled cutting devices for cutting wave shaped grooves in such articles las phonograph blanks, and wherein the signals to be recorded cover a very wide range of frequencies.

Another object is to provide improved signal-controlled apparatus for making extended frequency range phonograph master records, for example those used as masters to press multiple copies therefrom.

Another object is to provide improved constructions of electromagnetically operated cutting heads for engraving a record -blank .in accordance with electric signals of a very wide frequency range.

A feature of the invention relates to a cutting device having a vibratory cutting stylus with an improved mounting 'and excitation arrangement therefor, whereby the disturbing effects of natural resonance of the vibrating s'ystem can be placed well outside the upper frequency to be recorded. This feature is of particular importance in present day high fidelity or extended frequency records wherein frequencies up to 15,000 cycles per second are to be recorded.

Another feature relates to a cutting head for signal recorders generally, wherein a novel feed back element is embodied therein to provide electromagnetic damping for eliminating undesirable residu-al mechanical resonance effects.

Another feature relates to a signal transducer for translating electric vibrations into mechanical vibrations, :and

' comprising a vibrating conductor having a portion of substantially linear formation mounted in a magnetic field, and an angular extension which is swingably anchored to a rigid support, together with means to pass a signal current through said conductor to cause it to vibrate in pro portion to the signal current.

A further feature relates to a cutting recording unit comprising means to develop a steady magnetic eld, a longitudinally rigid conductor in said field, said conductor being swingably anchored at one end to a rigid support, the other end of said conductor being substantially directly united to a cutting stylus, land a rigid linear extension also attached to said stylus and having the opposite end swingably attached to a rigid support.

A further feature relates to an improved recording cutter of the stylus cutter kind, having ya magnet with' a substantially linear magnetic gap, a longitudinally rigid conductor to which signal currents are applied, said conductor having one end swingably -anchored to a rigid support to restrict the conductors vibration to a plane substantially parallel to the gap walls, the opposite end of the conductor being substantially directly attached to the y 3,020,361 Patented Feb. 6, 1962 ice disturbing resonance effects in the cutting frequency range.

A further feature relates to novel stylus cutting head of the heated stylus kind.l v

A still further feature relates to the novel organization, arrangement 'and relative location and interconnection of parts which cooperate to provide an improved stylus cutting head.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing which shows certain preferred embodiments,

FIG. 1 is an elevational view of a cutting head Iand system according to the known arrangement.

FIG. 2 is an elevational view of a cutting head and system 'according to the invention.

FIG. 3 is a modification of FIG. 2.

'FIG 4 is a sectional view of the cutting head shown in FIG. 3 taken along the line 4 4 thereof.

FIG. 5 is a sectional view similar to FIG. 4 of a modification of the magnet arrangement for producing the main magnetic eld 'and the supplementary or feed-back control field.

'cutting the record grooves in a blank Iare of limited response, because they generallyemploy a moving iron armature with a relatively long shank between the arm and cutting stylus. One typical construction of such conventional cutting heads is shown in FIG. 1. It usually comprises a permanent magnet 10 having opposed pole pieces 11, 12 which are cut to dene a lozenge-shaped vmagnetic gap in which is mounted for oscillation about its `center a loZenge-shaped magnetic armature 13. A driving winding 14 surrounds the armature and is connected to the source of electric signals to be recorded as cuttings. The armature 13 is provided with a relatively long shank 15 which carriesat its vlower end the cutting stylus 16. With such-an arrangement the stylus is vibrated in a plane parallel to the sheet of the drawing, namely in a plane parallel to the length of the magnetic lines of force in the magnetic gap, and it is clear that the gap width varies as the armature vibrates. Thus on one side the gap narrows as the armature approaches,rand at the opposite end it simultaneously widens. Even when the length of the shank is as short -as 2%; inch, lthere still exist resonance difficulties between the point of application of the driving force and the actual location of the stylus. Thus the shank takes no part in applying driving force, but on the contrary it increases the length of the vibrating system between the exciting windings and the stylus point. Even if the shank is made sufficiently short so that the mechanical resonance is above 15,000 cycles per second, that is only a compromise, because disturbing resonance effects are still present because of cross modulation inherent in tracing the groove. The present invention provides a solution for the above difficulties by employing anextremely light-weight moving system wherein the shank itself forms the excitation element and can be made of a high degree of longitudinal rigidity While providing extremely short coupling length between the point of application of the excitation or driving force, and the stylus point.

Furthermore, by using a simple linear conductor in a linear magnetic gap, the conductor can be mounted for cantilever motion at one end and is constrained to swing in a plane which is transverse to the lines of magnetic force. In other words the gap always remains of constant width on opposite sides of the moving conductor armature. Furthermore, since the moving armature '3 is non-magnetic it is not subject to such effects as hy'si teresi's or change of magnetization.

Referring to FIG. 2 which shows a cutting head and recording systeml according tb the invention, the dot-dash line represents the ilat surface f a phonograph blank wherein the record grooves are to be cut as wavy grooves of the so-called lateral cut kind. The head comprises a suitable permanent magnet i7, having a pair of pole pieces 18, 19 whose liat opposed faces 20, 21 define the magnetic gap. Since the pole faces are perfectly flat and parallel the magnetic eld as represented by the tine lines across the gap, can be designed with a high degree of uniformity throughout the gap length. These lines of force extend parallel to each other and parallel to the plane of the sheet of the drawing. The magnet with its pole pieces can be rigidly attached to any suitable rigid support (not shown).

Mounted in the magnetic gap is a metal strip 22 of a suitable lightweight non-magnetic metal such for example as beryllium. The strip 22 is bent at right angles to form a vertical arm 23 and a horizontal arm 24. A pointed cutting stylus 25 of any Well-known kind, for example a sapphire point, is fastened directly to the bent strip, preferably at or adjacent the bend 26. The stylus can be attached to the strip 22 by soldering and for that purpose its upper non-pointed end carries a suitable metal coating to facilitate soldering. One such metal coating may be a thin deposit of platinum obtained for example by reduction to metallic platinum from a solution of chloroplatinic acid. Instead of a platinum coating, a thin deposit of silver may be provided by ring a suitable silver paste at about 1100 degrees F In veither case a thicker copper plating is overplated on the first coating to permit tinning to facilitate soldering between the stylus and the strip 22. I have found that such a joint is necessary if the stylus is to be subjected to a heating operation during the recording as will be described hereinbelow in connection with FIG. 3. If no such heating of the stylus is required during recording, it can be attached tol the strip 22 by any suitable cement such for example as the well-known De Khotinsky cement.

The strip 22 may be of approximately 0.003 inch in cross-sectional thickness and approximately 0.10 inch wide. The upper end 27 of arm 23 is attened and twisted at right angles to the remaining length of the arm, and :that attened twisted end is fastened to a suitable rigid support 28. Thus the strip 22 is capableof cantilever vibration around the line 29 as a horizontal line of exure. Likewise the end 30 of arm 24 is attened and twisted around at right angles to the remainder of arm 24. The at end 31 is fastened in any suitable manner to a rigid support 31 so that the arm 24 is capable of swinging flexure around line 32 as a vertical line of flexur-e. The ends 27 and 30 of strip 22 areconnected to any well known source of electric signals 33 to be recorded. The arms 23 and 24 are rigidin the direction of their lengths and therefore by interaction between the magnetic field in the magnetic gap, and the signal current which ows through arm 23, the latter is vibrated around the flexure line 29 in a direction which is always transverse to the magnetic lines of force. The arm 23 does not change its spacing with respect to the iiat pole faces since it is longitudinally rigid. Likewise the arm 24 is longitudinally rigid and is constrained to execute a swinging movement around the exure line 32. The net result is that the stylus 25 vibrates in a direction perpendicular to the plane of the sheet of drawing and makes lateral cut grooves in the face of the phonograph blank.

I have found that with fthe foregoing arrangement of FIG. 2. and as described, because of the low mass of the strip 22, and the fact that it also acts as the electromagnetic armature Vto which the stylus is directly attached, it is possible to obtain vastly improved higher 4 frequency response. Usually the recording current from the source 33 is relatively large, for example of the order of tive amperes, but the electrical impedance of the moving system comprised of the strip 22 is only about 0.015 to 0.010 ohm, so that by using a suitable output transformer between the source 36 and the strip 22, it is possible to drive the stylus from conventional electron tube power amplifiers.-

l have found that even better results are obtained if means are provided for damping `out the residual effects of mechanical resonance of the moving system. Heretofore such damping has been eifected by using various combinations of rubber or rubber-like materials or special plastics as the damping elements. Such damping materials tend to change their damping characteristics with aging and with temperature changes. I have found that the objections to such dampers of the buffer kind, can be avoided by employing electronic-mechanical damp-` ing and by using an electronic-mechanical damper of very small mass which can be directly and closely coupled mechanicallypto'the main driving element, such as that describedhereinabove in connection with FIG. 2. With such `a combination itis 'possible to remove the disturbing resonance frequency far beyond that obtainable heretofore. Y Y

FIGS. 3 and 4 show such a combination of elements wherein two separate magnetic gaps are provided as indicated by the numerals '34, 35'. For that purpose two separate magnets 35, 37 are provided. While those magnets may be of any suitable shape, preferably they are formed as shown in the drawing. Thus magnet 36 may comprise a straight magnetic bar 3S which is joined to an L-shaped magnetic bar 39 with an intervening permanent magnet yoke 40, thus deiining the magnetic gap 34. Magnet 36 can be fastened to a bar 41 'of brass or other non-magnetic material which provides a com'- mon support for bothmagnets, and also acts as a shield to reduce alternating current coupling between the two moving systems. Similarly magnet 37 comprises a straight magnet bar 42 and an L-shaped magnet bar 43 joined to the permanent magnet yoke 44, and dening the magnetic gap 35. The magnet 37 is fastened to the brass plate 41 adjacent the lower end thereof and as shown in FIG. 3.

Since the gap 34 is the main gap, its length L may be materially greater than the length 1 of gap 35. In, order to minimize stray coupling, the magnets 36, 37 f are so designed that they have their like poles adjacent, as shown in FIG-.4. If necessary, the two magnets can be completely magnetically shielded from each other by any well known magnetic shielding casing or the like.

Attached to suitable rigid supports 45, 46 which may be the same member, is a moving system similar to that described hereinabove in connection with FIG. 2. It consists of a vertically arranged non-magnetic light-Weight metal strip or arm 47, and a horizontally arranged nonmagnetic light-weight metal strip or arm 4S, Ithe arms 47 and 48 being substantially `at right angles. If desired, the arms 47 and 48 may be made from one length o-f material bent at right angles as in FIG. 2. However, preferably, they are separate so that they can be soldered at their respective ends to different facesor sides of the cutting stylus 49. In order to increase the longitudinal rigidity of the arms 47, 48 they may be made of metal strip which is slightly hollow in cross-section as shown in FIG. 4. The metal strip stock can be of the order of 0.003 inch in cross-sectional thickness, and about 0.10 inch wide.

The upper end of arm 47 is ilattened and twisted at right angles and riveted or otherwise rigidly fastened flat against the support 45 so as to enable the arm 47 to swingably iiex along the sharply defined horizonal flexure line 50, without changing its centralized position in gap 34. If necessary the flattened end of arm 47 can be clamped between two plates or blocks so as to dene more acinsulate the flattened ends of the arms 47, 48 from their supports. However, if desired the supports 45, 46 themselves may be of insulating material. The elements 47, 48 constitute the main moving system which is energized by the amplified signal output from the driving amplifier 54, thus driving the stylus 49 which is therefore constrained to vibrate in a plane perpendicular to the plane of the sheet of drawing. This effects a lateral cut groove in the surface of the record blank 55.

In order to damp out residual resonance effects there is attached to the arm 48 at approximately its middle, another vertical arm 56 of non-magnetic light-weight material such for example as beryllium, and which is located in the gap 35. The upper end of arm 56 is flattened and twisted at right angles to form a flat anchoring lug which is riveted or otherwise anchored to a rigid support -7 which may be integral with the supports 45, 46. Thus the arm 56 is capable of swinging flexure around the sharply definedghorizontal flexure line 58. The lower end of arm 56 is connected over conductor 59 to one terminal of an electrical network 60 which is of the kind which produces output voltages for given input voltages and the said output voltages drop olf in amplitude at the higher end of the frequency band. The upper end of arm 56 is connected by conductor 61 to another terminal of network 60. It will be observed that the network 60 is connected between the source 62 of the electric signals to be recorded and the input of the amplifier 54.

As the stylus 49 is being driven by arm 47 to execute its cutting of the record 55, the arm 56 is driven in unison therewith since it is mechanically and substantially directly coupled to arm 47. Thus by the interaction between the moving arm 56 and the magnetic field in gap 35 there are generated feed-back voltages. The magnet 37 with member 56 constitutes a feed-back voltage generator which provides a negative feed-back voltage at the signal frequency, and this feed-back voltage is applied to the input of the amplifier 54 in the proper negative phase with respect to the phase of the signals from source 62, so that the over-all loop transfer characteristic of the system between the .input and output of amplifier 54 is prevented from encompassing the point -l,0 in the complex impedance plane plot of the transfer characteristic :impedance as explained in detail in U.S. Patents No. 2,131,365 and 2,102,671. Briefly the network 61B limits the amount of gain in the feed-,back loop between the output and input of amplifier 54 to a reasonable value,

` and reduces the feed-back gain at the higher frequencies before resonance of the moving system `47, 48 takes place. This function can be supplemented by designing the amplier54 so that its normal forward gains drops oil at the high end of the frequency band, as is well known in the electron tube amplifier art. The elements 37, 56 therefore constitute an electronic-mechanical damping device, and cooperation with elements 36, 47 maintain the natural resonance of the stylus driving system substantially higher than the highest frequency to be recorded, for example 15,000 cycles per second. One of the reasons that the damping is so effective is that the mechanically coupled feed-back arm 56 is directly and closely coupled to the main driving arm 47, and can provide a closed negative feed-back voltage system which is effective over a much wider range than has been found practicable heretofore.

In order to increase the speed at which the stylus 49 `can cut the groove in record blank 55, the said stylus 65 to a suitable source 66 of heating current. The wires 64, 65 and the heating coil should preferably be plated Nichrome or some similar material.

With the foregoing described combination of elements it is possible to cut a master record blank with the greatly extended frequency band width, at a cutting speed which is at least twice the normal cutting speed. For example if the source 62 is a magnetized tape record of the signals, it can be run at twice its normal speed, as can the master record blank 55. This materially reduces the cost of making such master records.

FIG. 5 shows a modified arrangement of the dual magnet system similar to that of FIG. 3, except that each magnet system is completely closed magnetically. Thus as seen in FIG. 5 the magnet 36A comprises a pair of permanent magnets 67, 68 which are joined by two soft iron bars 69, 70. Centrally attached to bar -69 is a soft iron pole piece 71 which at its flat front face defines the main magnetic gap 34 which cooperates with the stylus driving arm 47 as described in connection with FIG. 3. Likewise the magnet 37A comprises a pair of permanent magnets 72, 73 joined by soft iron bars 74, 75. Bar 75 carries the soft iron pole piece 74 whose flat face defines the auxiliary or feed-back magnetic gap 35 which cooperates with the feed-back generating arm 56 as described hereinabove. The two closed magnets can be fastened to a plate 76- of brass. If desired the plate 76 can be laminated with high permeability iron sheets to complete the magnetic shielding between the two magnet systems.

While the invention has been described in connection with the recording of'sound or music, it will be understood that it is capable of recording any kind of signals having a wide frequencyk range. For example it is possible to make practical use of the inventive features in making records of industrial programming or automation of machines, processes, and the like. It can be used for the recording of telemetering data from missiles, aircraft and other fast moving objects. Various changes and modifications can be made in the disclosed embodiments Without departing from` the spirit and scope of the invention.

What is claimed is:

l. An electromechanical transducing device of the kind having a stylus in transduciug engagement with a moving surface, comprising means defining a magnetic gap, an varmature of non-magnetic material located in said gap and arranged to be excited by electric currents tocause Vibration of said armature, a stylus connected to said armature, means supporting said armature to constrain it to execute vibrations in one plane substantially perpendicular to the magnetic lines of force across said gap while constraining said stylus against torsional vibration with ture and having flexibility in another plane only and normal to said first plane.

2. An electromechanical engraving device of the kind for engraving lateral cuts in a moving record blank, comprising means defining an elongated magnetic gap, an armature of non-magnetic material located in said gap, an engraving stylus attached to said armature adjacent one end of the armature, means supporting said armature at its opposite end at a point outside said gap to constrain the armature and stylus to vibrate in a plane substantially normal to said blank, and additional support means also connected to said stylus and armature to constrain said stylus lagainst vibration in a plane normal to the first mentioned plane and also against torsional vibration about an axis parallel to `said record blank, the first mentioned supporting means comprising alongitudinallyrigid arm connected to the armature and having flexibility in only acarrear 7 one plane normal to said blank, and another longitudinally rigid flexible arm also connected to said armature and having iiexibility in another plane only and normal to said first plane.

3. An electromechanical transducing device of the kind having a stylus and a moving surface in contact therewith, comprising means defining a magnetic gap, an armature located in said gap and constituted of a longitudinally ngid non-magnetic conductor, a stylus attached to said conductor adjacent one end, means supporting said conductor at its opposite end at a point outside said gap, and additional support means also outside said gap and attached to said stylus to maintain said armature conductor substantially centralized in said gap, each. support means being Aa longitudinally rigid arm and each arm being attached to a respective rigid member, respective tiexible joints between each arm and its rigid member with the joints having respective flexibility only in mutually perpendicular planes to constrain said stylus to vibrate substantially entirely inone plane substantially perpendicular to the movement of said surface while constraining said stylus against torsional vibration around an'axis parallel to said surface. i

4. An electromechanical engraving device for engrav ing a moving record surface, comprising means deiining a magnetic gap, a vibratory signal-energized moving armature comprised of a first longitudinally rigid conductor located centrally of said gap, a second longitudinally rigid conductor located outside said gap and being substantially transverse to theirst conductor and connected thereto, a stylus connected to said conductorsfor vibration there with, means separately supporting said conductors at their respectiveunconnected ends', one supporting means being a longitudinally rigidarm having flexibility in only one plane normal to said record surface enabling the first conductor to vibrate only in one plane perpendicular to the said surface, the other supporting means also being a longitudinally rigid arm having flexibility only in a plane normal to the irst mentioned plane to enable the second conductor to vibrate only in one plane perpendicular to the first mentioned plane, both said supporting means being attached to their ilexible portions to respective rigid supports to constrain said stylus against torsional vibration around an axis parallel to said surface.

5. A transducing device according to claim 4 in which said first and second conductors are constituted of a single longitudinally rigid metal strip which is bent substantially at right angles.

6. A transducing device according to claim 4 in which said first and second conductors are longitudinally rigid and are united to form a substantial right angled member, and a cutting stylus is attached to one of said conductors.

7. An electromechanical engraving device for engraving a moving surface, comprising means defining a main magnetic gap and an auxiliary magnetic gap,l an armature located in said main gap and arranged to be excited by electric signal voltages to cause said armature to vibrate in said main gap, an engraving stylus carried by said armature, means supporting said armature to constrain it to execute vibrations in one plane substantially perpendicular to said surface and constraining the armature against vibrations in another plane perpendicular to the tirst inentioned plane to prevent torsional vibration of said armature and stylus with respect to said surface, and another armature located in said auxiliary gap and mechanically connected to the rst armature for vibration in unison therewith to generate separate control voltages for reaction with said signal voltages, the means for supporting and constraining the armature to execute vibrations in said one plane comprising a first longitudinally rigid flexible arm having flexibility in only one plane, and another longitudinally rigid exible arm having flexibility in another plane only normal to said first plane.

`3. An electromechanical transducing device for engraving a moving surface, comprising means defining a main magnetic gap and a magnetically separate auxiliary magnetic gap, an armature for the main gap comprising a longitudinally rigid conductor supported at one end by a joint which is capable of iiexure substantially only around an axis which is substantially parallel to said surface,- longitudinaily rigid means connecting the opposite end of said armature conductor to a rigid support offset with relation to the length of said armature conductor to provide a flexural axis for said rigid means which axis is substantially at right angles to the irst mentioned flexural axis, whereby the vibrations of said armature conductor are coi-dined substantially entirely to a single plane normal to said surface and said armature conductor is substantially free from torsional vibration, and another and separate armature conductor attached at one end to said rigid member and at the other end to a rigid support by a joint which provides a exural axis for said separate armature which is substantially parallel to said surface.

9. A transducing device according to claim 8 in which a pointed engraving stylus is fastened to said rigid member for vibration in unison with the first mentioned armature.

l0. An engraving device for engraving the surface of Va record blank moving in a given plane, comprising means deliningan elongated magnetic gap, a longitudinally rigid thin metal strip located substantially centrally along said gap, a lirst rigid support, Ka iirst exural joint between one end of said conductor and said rigid support and having substantially a single exural axis which is transverse to the width of said strip, a second rigid support, a second iiexural joint between said second rigid support and the opposite end of said longitudinally rigid means and having a substantially single exural axis which is transverse to the rst mentioned iiexural axis whereby said metal strip is constrained against torsional vibration, and an engraving stylus fastened to said strip for vibration in unison therewith.

ll. An engraving device according to claim 10 in which the said strip is attened at one end and is twisted approximately at right angles to provide the first mentioned liexural joint, and the said longitudinally rigid means is also in the form of a metal strip having one end flattened and twisted at approximately right angles to provide the second mentioned flexural joint at substantially right angles to the first mentioned flexural joint.

l2. An engraving device according to claim l0 in which means are provided for deiining a magnetically separate auxiliary magnetic gap, and another longitudinally rigid thin metal strip is located substantially centrally along said auxiliary gap, the last mentioned strip being fastened to said Vlongitudinally rigid means and provided at one end with a flexural joint whose exural axis is substantially parallel to the first mentioned flexural axis.

13. An engraving device for engraving a record in a blankmoving in a given plane, comprising magnetic means defining an elongated magnetic gap, a. longitudinally rigid thin metal strip located substantially centrally along said gap, means iiexurally supporting said strip at one end for vibration in a plane substantially parallel to the width of the strip, longitudinally rigid means connected to the opposite end of said strip and flexurally supported at its opposite end for vibration in unison with said strip, a pointed cutting stylus carried by said strip for vibration therewith, means defining an auxiliary magnetic gap which is substantially parallel to the first mentioned gap, another longitudinally rigid thin metal strip located substantially centrally along said auxiliary gap and fastened to said longitudinally lrigid means, first lead-in means for supplying excitation currents to the metal strip in the iirst mentioned gap, and second lead-in means for leading olf the currents generated in the metal strip in the auxiliary gap.

14. Engraving apparatus of the kind having an engraving stylus and a movable surface to be engraved thereby, comprising an electric signal amplifier having input terminals for connection to a source of electric engraving signals, means defining a main magnetic and a separate auxiliary magnetic gap, a nonfnagnetic arrnature mounted for swingable vibration in said main gap, means connecting output terminals of said amplifier to the armature, another armature mounted in said auxiliary gap, means mechanically joining both said armatures, circuit connections between the second armature and the input terminals of said amplifier to dampen undesirable resonant vibrations of the first armature, an engraving stylus substantially directly attached to said first armature, and means supporting said armatures to constrain said stylus to move substantially parallel to said surface while constraining the stylus against movement in the direction of the surface movement, the said supporting means including a flexural support for the first armature arranged substantially at right angles to the movement of said stylus to constrain the first armature to swingably vibrate substantially centrally of said main gap.

15. Engraving apparatus of the kind having an en graving stylus and a moving surface to be engraved thereby, comprising means defining a main magnetic gap and an auxiliary magnetic gap, a main stylus driving armature located in said main gap, an engraving stylus carried by said armature, an auxiliary armature located in said auxiliary gap, means supporting said main armature at opposite ends through respective exural joints Whose respective iiexural axes are substantially at right angles whereby the said main armature is constrained against torsional vibration while vibrating said stylus in a path transverse to the movement of said surface and constraining said stylus against movement except in said transverse direction, an electric signal amplifier having input terminals connected to a source of electric engraving signals, circuit means connecting the output of said amplifier to said main armature to drive said stylus and simultaneously to vibrate said auxiliary armature to produce negative feed-back voltages, and additional circuit means connecting said -auxiliary armature to the input of said amplifier to apply said feed-back voltages thereto.

16. Engraving apparatus according to claim in which the last mentioned means includes an electrical transmission network having a transmission versus frequency characteristic to produce an output voltage therefrom which drops od in amplitude at the higher end of the band of signal frequencies applied thereto.

17. An electromechanical transducer for engraving a moving surface under control of engraving signal currents, comprising means defining a substantially linear magnetic gap, an L-shaped metal armature having one of the legs thereof located in said gap and constituted of non-mag netic material, a stylus fastened to said member between the ends thereof but outside said gap, a first rigid support adjacent one end of said L-shaped member, another rigid support adjacent `the opposite end of said L-shaped member, means anchoring one end of said member to the first mentioned rigid support through a exural joint having a predetermined flexural axis, means anchoring the opposite end of said L-shaped member to the other rigid member including another flexural joint having ta flexural axis which is transverse to the first mentioned axis whereby the movement of said stylus is confined entirely to a path which is transverse to the direction of the movement of said surface and the said L-shaped member is constrained against torsional vibration.

1S. An electromechanical engraving transducer according to claim 17 in which said stylus is fastened to said member adjacent the angle of the L-bend thereof.

19. An electromechanical transducer comprising means defining a substantially linear magnetic gap, an L-shaped metal member having one of the legs thereof located in said gap and constituted of non-magnetic material, a stylus fastened to said member between the ends thereof but outside said gap, a heating element carried by the stylus, means anchoringone end of said member along one predetermined iiexural line, and means anchoring the opposite end of said member along another flexural line which is transverse to the first mentioned line.

20. An armature and stylus assembly for an electromechanical engraving device, comprising a thin metal strip of non-magnetic material bent to L-shape with one leg of the L located in a different plane from the other leg of the L, a rst rigid support, a second rigid support, the end of one leg of the L being twisted at an angle and attached to the first support to provide a flexural axis in a predetermined direction, the end of the other leg of the L being twisted at an angle and attached to the second rigid support to provide a flexural axis substantially at right angles to and located in a plane at right angles to the first exural axis, and a stylus fastened to said strip adjacent the angle ofthe L-bend therein.

References Cited in the file of this patent UNITED STATES PATENTS 1,916,162 Parker Jan. 27, 1933 1,960,007 Harrison May 22, 1934 2,161,489 Vieth et al June 6, 1939 2,162,986 Wiebusch .lune 20, 1939 2,182,136 Ricchiardi Dec. 5, 1939 2,239,717 Hunet et al Apr. 29, 1941 2,373,181 Fleming Apr. 10, 1945 2,413,971 Heintz Jan. 7, 1947 2,475,200 Roys July 5, 1949 2,746,130 Davis May 22, 1956 2,854,529 Williamson Sept. 30, 1958 

